This invention relates to a bending matrix for die bending sheet metal on folding presses. More particularly, the invention is concerned with a matrix having a profiled opening provided with a liner covering the profiled opening.
In the die bending of sheet metal on folding presses, the sheet metal is inserted between a ram and the matrix and is profiled by cold working. The ram acts as a die tool. When cold working various metals, the surface of such metals are marred. This is particularly so in the case of sheets of light metal, nonferrous metal, chromium steel, and surface-finished metal sheets (anodized, varnished, plastic-coated, plated, electroplated, hot-galvanized, etc.), because undesired friction points, scratches, or grooves occur during the bending process. Such markings must subsequently be removed, for example, by grinding or polishing.
To prevent such undesired markings, it is known to insert sheets of fabric or rubber between the sheet metal and the matrix. Foil-coated metal sheets can also be used. When such foils are used, they have to be removed after the stamping process. However, these extra steps in the processes are relatively expensive and labor-consuming.
It has therefore been proposed to use a single-piece plastic or steel matrices with a plastic insert completely surrounding the matrix opening. However, matrices of this type have not proved acceptable, since the plastic was excessively deformed by the high ram pressure. Such excessive ram pressure usually lead to inaccurate longitudinal profiles.
Since the injuries and damages to the sheet metal occur primarily at the inlet edges of the matrix opening, attempts have heretofore been made to prevent the injuries to the sheet metal by polishing these edges. However, it was found that scratches appeared again after only a few stampings, so that the polishing operation had to be repeated regularly. This repeated polishing naturally greatly hindered a rapid sequence of operations.
It is therefore an object of the invention to provide a bending matrix in which the afore-mentioned disadvantages are avoided.
Another object of the invention is to provide a folding matrix which makes use of a die bending tool in which the material to be profiled is free of undesired markings.